The present invention drawn to a bond coat system to be applied to a silicon containing substrate.
Silicon-based monolithic ceramics such as, silicon carbide and silicon nitride, and composites such as, silicon carbide fiber reinforced silicon carbide matrix are attractive materials for use in gas turbine engine hot sections due to their high temperature mechanical and physical properties as well as lower density than metals. However, these materials exhibit accelerated oxidation and recession in high temperature, aqueous environments such as, for example, the combustor and turbine sections of gas turbine engines. In order to reduce the rate of oxidation and recession at high temperatures in substrates used as ceramic components in such environments, significant effort has been given to providing barrier layers for the protection of the silicon based substrates so as to increase the service life of such components.
One variant of a protected coated ceramic article that constitutes prior art is shown in FIG. 1a. A composite article 10 comprises a silicon based substrate 12, a bond coat or layer 14 which comprises a dense continuous layer of silicon metal, and an environmental barrier coating comprising a barrier layer 16 which comprises either an alkaline earth aluminosilicate based on barium and/or strontium; or yttrium silicate. Another refractory top layer 18, such as, for example, aluminum oxide; or zirconium oxide; or yttrium oxide; or combinations thereof may be used on top of the barrier layer 16. Another embodiment of the prior art is shown in FIG. 1b where intermediate layers 20 and 22 are included for lending compliance and/or chemical compatibility to the coating system. The intermediate layer comprises, for example, a mixture of the barrier layer material with an additional oxide such as mullite. These prior art coating systems have proved to be quite adherent and protective and have been used to prevent recession and oxidation of the silicon based substrate 12. However, it has now been found that when the coatings depicted in the prior art are applied to some silicon-containing substrates such as silicon nitride, their mechanical properties suffer as demonstrated by a reduction in the flexure or tensile strength. It is believed that a loss in mechanical properties results from mechanical incompatibilities between the bond coat 14 and silicon-containing substrate 12. It has been found that a mismatch in elastic modulus and thermal expansion coefficients between bond coat 14 and the silicon-containing substrate 12 results in thermal strains in the bond coat 14 and silicon-containing substrate 12 when the coated system 10 is subject to thermal excursions such as cooling from the annealing temperature and/or heating and cooling cycles during gas turbine engine operation. It has also been found that the stored elastic strain energy required for crack propagation would be high in such systems. It is further found that failure in coating systems and brittle ceramics can occur due to high strains and stresses and/or high elastic stored energy in the system. Due to the brittle nature of silicon bond coat and silicon-containing substrate, the thermal strains and the resulting thermal stresses can lead to cracking in the bond layer or substrate or cause delamination of the interface between the bond coat and substrate. High thermal stresses may also lead to local stress concentrations and activate small defects or flaws in the bond coat or silicon-containing substrate leading to a failure well below the typical substrate strength.
Based upon the foregoing, it is clear that the need remains for an improved bond coat system for silicon based ceramic substrates that does not adversely affect the mechanical behavior of the substrate such as reduction in the flexure or tensile strength. It is therefore the primary object of the present invention to provide such a coating system architecture.
Accordingly, it is a principal object of the present invention to provide a bond coat system which does not adversely affect the mechanical properties of silicon based substrates such as silicon nitride. The bond coat system of the invention incorporates the advantages offered by a silicon metal containing bond coat and barrier coating applied thereto while at the same time overcoming the disadvantages noted above.